The present invention relates to a filter plate-and-diaphragm assembly for a plate or frame filter press of the type including a filter plate provided with a depressed base and a thickened peripheral portion defining a circumferential sealing edge, and a pressure diaphragm provided on each side of the plate and having a cup shape in order to conform to the shape of the depressed base of the plate.
In known diaphragm plates, the pressure diaphragm is connected with the plate or to a corresponding frame by means of an adhesive or by vulcanization. Replacing the diaphragm without damaging the wall or frame portions is almost impossible and is, in any event, very time consuming.
German Offenlegungsschrift [Laid-open Application] No. 23 22 044, and corresponding U.S. Pat. No. 3,931,014 to Heimbach et al, discloses a cup-shaped diaphragm which has a dovetail-shaped edge via which it is fixed in a corresponding groove of the plate. This construction makes it possible to achieve a good yield from the filter cakes even in the edge regions and to operate successfully with locking forces which are as low as possible for the packet of plates formed from a plurality of such assemblies. Since the dovetail joint must fit precisely between the pressure diaphragm and the plate, its manufacture is relatively expensive.
For the purpose of facilitating connection of the plate and the pressure diaphragm, German Utility Model Pat. No. 19 85 323 discloses an overhanging diaphragm which rests on the upper edge of the plate, with no firm connection of the diaphragm edge with the plate edge, and hangs down smoothly toward the bottom. Since no special fastening elements are provided, such a diaphragm cannot be used as a cup diaphragm as it is liable to be displaced from its position with respect to the plate when the filter press is opened. Therefore, all plates must be checked to see that the overhanging diaphragms are in their correct position before the plate packet of the filter press in question is closed. Automatic operation is thus practically impossible.
German Utility Model Pat. No. 69 06 156 discloses another overhanging diaphragm which is also planar but which, in contradistinction to the above-described known overhanging diaphragm, has its smooth edge region clamped into a frame which is connected with the plate so that, during opening of the filter press, the plate, diaphragm and frame can be displaced as a unit. Since the diaphragm is planar, the diaphragm material will be stretched considerably along the inner clamping edge when it is charged with pressure medium from the rear. Furthermore, due to the planar design of the pressure diaphragm, the filter cake will not be sufficiently pressed in its edge regions. This solution is expensive from a production point of view since, for each diaphragm, the clamping edge of the plate as well as that of the clamping frame must be machined to be perfectly planar and parallel to one another. Also, replacement of such a diaphragm is made mroe difficult in that initially the clamping frame must be removed from the plate before the diaphragm can be lifted off.